A typical bi-direction motor speed control relies on an arrangement of switching devices called an H-Bridge. The H-Bridge arrangement allows current through the motor to be steered in one of two directions permitting forward or reverse rotation of the motor. In the H-Bridge arrangement, current in the motor flows through two switching devices at all times. Each switching device causes a loss in efficiency. Typical brushed motor speed controls use a solid state H-bridge to reverse the voltage across the motor. This H-bridge can also be used for motor braking. Unfortunately, the H-bridge causes two switching devices to be in series with the motor which can reduce the operating voltage, increase power dissipation, and decrease efficiency. In many applications, reverse rotation of the motor does not need to have the same performance as forward rotation of the motor. One example is in an electric car. In these cases, forward rotation is much more important to provide the maximum efficiency than for an auxiliary reverse rotation.
Referring to FIG. 1, depicted is a schematic block diagram of a prior technology H-bridge for controlling a motor in forward and reverse directions. A switching configuration for controlling current direction through a motor 102, (i.e., for switching voltage polarities at the motor 102 terminals) comprises power transistors 102, 104, 106 and 108 arranged in an H-bridge configuration. The power transistors 104 and 110 are adapted to couple the motor 102 to a voltage source 112 (VBUS) and the power transistors 106 and 108 are adapted to couple the motor 102 to a return (ground or common) 114 of the voltage source 112, e.g., battery (see FIG. 7), depending upon the desired direction of rotation of the motor 102. The power transistors 102, 104, 106 and 108 may be controlled on and off by control signals at gate inputs 124, 126, 128 and 130, respectively.
Referring to FIG. 2, depicted is a schematic block diagram of the H-bridge of FIG. 1 configured for the forward direction of the motor. When it is desired for the motor 102 to rotate in a forward direction (e.g., arbitrarily selected herein for illustrative purposes), the power transistors 110 and 106 are switched on by the control signals 130 and 126, respectively, and the power transistors 104 and 108 are switched off by the control signals 124 and 128, respectively. This switched configuration will allow current to flow through the motor 102 in a direction 250 (also indicated by heavier lines of the schematic).
Referring to FIG. 3, depicted is a schematic block diagram of the H-bridge of FIG. 1 configured for the reverse direction of the motor. When it is desired for the motor 102 to rotate in a reverse direction (e.g., arbitrarily selected herein for illustrative purposes), the power transistors 104 and 108 are switched on by the control signals 124 and 128, respectively, and the power transistors 110 and 106 are switched off by the control signals 130 and 126, respectively. This switched configuration will allow current to flow through the motor 102 in a direction 350 (also indicated by heavier lines of the schematic) opposite to the current flow direction 250 shown in FIG. 2.
By switching the current flow to either direction 250 or direction 350 (i.e., reversing the motor 102 terminals coupled to the voltage source 112 and the voltage source return 114) the motor 102 rotation may be reversed. However, there are always two of the power transistors in the current flow path, thus creating power losses through these two power transistors.